Effective factors for appearance or non-appearance of Chloritoid mineral in the metapelitic rocks of Bolandparchin and Poshtuk area

Microstructural and petrographical features as well as field relations, show that formation of the metapelitic rocks of Bolandparchin and Poshtuk in northwestern Iran was poly-metamorphic. Mineral assemblages in the metapelites of two areas are different. The Bolandparchin schists are graphite-rich and have dark color. Peak metamorphic assemblage is garnet, biotite, staurolite, aluminosilicate minerals (andalusite/sillimanite), muscovite and quartz in these rocks, but Poshtuk metapelites are graphite-free and their mineral assemblages are garnet, staurolite, chloritoid, chlorite, muscovite and quartz, which usually form by contact metamorphism. The isograds are not compatible with Barrovian type, but reflect the first appearance of chloritoid in the aluminous bulk compositions. This study shows that in spite of same protolith and geothermal gradient in the both areas, existence of different mineral assemblages is due to presence or absence of graphite and consequently different aH2O, so that Poshtuk rocks are graphite-free and have aH2O ~ (0.8-1.0); therefore, chloritoid is formed. Bolandparchin rocks have aH2O ~ (0.5) and aluminosilicate minerals such as andalusite/kyanite/sillimanite are formed in these rocks.